Antibodies in the blood are highly stable, and since they have no antigenicity, they are drawing much attention as pharmaceuticals. Of these, it has been a while since bispecific antibodies, which can simultaneously recognize two types of antigens, have been proposed, but only those that merely bind two types of antigens exist at present. However, since antibodies bind to specific epitopes within antigens, it may be possible to place two antigens at desirable distances and angles by selecting appropriate antibody combinations.
In many cytokine receptors, it is thought that the angle and length of chains that form homo/hetero-dimers change when a ligand binds, thus enabling the receptors to transmit signals into cells. Thus, appropriate anti-receptor antibodies can mimic receptor dimerization initiated by ligand-binding, and become potential agonistic antibodies. Monoclonal antibodies that display agonistic activity against MPL, a homodimer, have already been reported (Blood 1998 Sep. 15; 92(6): 1981-8, US98/17364). However, to obtain such agonistic antibodies, selection must be made from a huge range of antibodies, requiring effective selection methods.
In conventional assays, it is necessary to select antibodies that bind to antigens, i.e., receptor chains, and add these antibodies to an appropriate cell assay system that responds to the ligands. This becomes particularly troublesome where the receptors form heterodimers. Antibodies against each of the two chains (A, B) that form the receptor must be selected, and every combination of A and B must be tested one by one. In addition, to generate bivalent antibodies, it is necessary to fuse antibody-producing hybridomas, or to construct expression vectors for all of the antibodies, and introduce all combinations thereof into cells. Examination of 100 types of antibodies against each of the A and B chains necessitates the testing of 10,000 types of combinations, and requires a total of 400 types of expression vectors for L and H chains to be constructed and introduced into cells 10,000 times. There are also methods that use libraries that provide antibodies displayed on phages as bispecific diabodies. However, since the direct addition of E. coli culture supernatant to cell culture systems has a bad effect on cells, purification becomes necessary and monospecific diabody contamination (theoretically 50%) becomes inevitable.